Best thing that worked for my "heavy" legs was diving with a drysuit where I can get just a little bit of air in my feet to keep them from sinking (Disclaimer: of course always be super careful when moving air into your feet in a drysuit as you can't simply vent that air and therefore you need to be aware of the techniques for doing so. Disclaimer over). In a wetsuit, however, it's a struggle for me with the sinking feet (despite my core being fine). I've been trying to balance (no pun intended) the "moving the tank up for trim" versus constantly banging my head on the tank valve, and haven't come up with a good solution yet.
Other than that, if you're still struggling with all the great info already given, and as dumb as it sounds, you could always add a couple of small blocks of Styrofoam to your fins to make them less negative.